Capacitive PD Sensing and Transmission Line Modeling for Long-Range Cable Defect Detection

High Voltage conductor, thus requiring an outage to be installed. For this reason, while they are widely adopted in laboratories and off-line testing, traditional coupling capacitors cannot be deployed for on-line testing. This study demonstrates the effectiveness of non-invasive active capacitive sensors in detecting and localizing PD over several kilometers, overcoming the limitations of conventional galvanic or inductive sensors. Non-invasive capacitive sensors operate by coupling to the electric field induced by transient PD events, capturing highfrequency components without galvanic contact. However, as distance increases, the distributed parameters of the cable—particularly its low-pass filter behaviour—degrade signal amplitude and temporal resolution. To validate sensor performance under realistic conditions, a field test was conducted on a 35 kV, 3 km long solar farm cable exhibiting suspected splice defects. To support interpretation of field results and improve the knowledge on the PD signal propagation through coaxial cables, a distributed parameter transmission line model was developed. This model simulates PD pulse propagation, accounting for per-unit-length resistance, inductance, capacitance, and dielectric loss mechanisms. Simulation results closely matched measured data in both time and frequency domains, reproducing characteristic attenuation and dispersion effects. Importantly, the model allowed prediction of expected signal degradation profiles, facilitating the estimation of detection limits and optimal sensor spacing. The combination of non-invasive sensing and simulation-based interpretation represents a robust framework for a condition-based cable monitoring and enabling defect localization. This work confirms that capacitive sensors can effectively detect PD over distances up to several kilometers, even in systems with complex layouts or embedded joints.